Man-made structures have experienced nature’s fury time and again. We look at how Kedarnath temple has withstood the test of time, and how IIT Madras is facilitating the restoration of such heritage structures.
I remember my first visit to the Himalayas, standing thousands of feet above sea level. It was a spectacular view – dense green forests, massive rock cliffs, chains of silvered peaks and grasslands on the banks of the Mandakini river that ran deep and silent. Just imagining that the ground below me while standing there starts shaking at an acceleration approximately equal to one-fourth of that of a free falling object gives me goosebumps. Although qualitatively similar, it certainly is not a ride at an amusement park. This exact phenomenon has happened in the region around Kedarnath temple multiple times, the most recent one being in 2013.
The temple is located near the Chorabari Glacier, one of whose two noses is the source of Mandakini River. This terminates at Chorabari Lake which is approximately 400 metres long, 200 metres wide and 15-20 metres deep and is situated about 3 kilometres upstream from Kedarnath. On June 16, 2013 heavy rainfall together with the melting of the snow from the glacier led to the complete draining of the lake within a matter of minutes. Once the lake had burst, the water carried along mud and debris down the valley and caused massive devastation to the entire town downstream.
However, there was no major damage to the temple structure. It is immensely strong, made of thick, massive granites and high-grade ‘metamorphic gneissic’ rock slabs, pillars and bricks. Simply put, these are rocks that changed form due to heat and pressure while buried deep below the earth’s surface, have banded appearance and are made up of mineral grains, typically quartz minerals. They have a bright sheen, are rough to touch and very hard. The walls pillars are about 2.6 metres thick. The roof is an assemblage of multiple blocks with dressed stone on the exterior. This is one of the major reasons that the temple withstood the earthquakes. The survival of the temple can also be attributed to the presence of a man-made platform which raises the temple super-structure that prevented the temple from the direct flow of gushing floodwater.
India has one of the largest stocks of heritage structures in the world out of which 32 are formally recognised by the United Nations Educational, Scientific and Cultural Organisation (UNESCO). Of these sites, 25 are cultural properties and 7 are natural properties. Formal systems that recognise conservation of heritage structures as a multidisciplinary engineering efort do not exist in India. Heritage conservation doesn’t mean freezing a building in time or creating a museum. Instead, it seeks to maintain and thereby increase the value of buildings by keeping their original architectural elements, favouring their restoration rather than replacement and, when restoration is impossible, recreating scale, period and character. Addressing the task of understanding and protecting heritage structures from natural hazards, ageing and weathering effects is a serious problem in India supplemented by the lack of adequate quality and quantity of manpower.
With the intent of beginning a formal approach to address the safety of heritage structures, the National Centre for Safety of Heritage Structures (NCSHS) was established at IIT Madras in July 2013 with Dr. Arun Menon, a professor in the Structural Engineering Laboratory at the Department of Civil Engineering, designated as the Convener. Dr. Menon tells me that NCSHS is envisioned as a long-term programme towards addressing the challenge of ensuring structural safety of historical monuments and other heritage structures in India. The plan is to collaborate with implementing agencies such as Archaeological Survey of India (ASI) which would help in fundamental research and education.
Dr. Menon’s primary research interest is the seismic vulnerability assessment of building structures. The Indian subcontinent has a high seismicity – the frequency of earthquakes in a region. The country has been divided into 4 seismic zones (Zone 2, 3, 4 and 5) where Zone 5 and Zone 2 expect highest and lowest levels of seismicity. Kedarnath temple, situated on the Garhwal Himalayan range near the Mandakini river in Uttarakhand, stands in Zone 5. Its neighbourhood has seen several earthquakes in the recent past such as in 1991 (Uttarakashi) and 1999 (Chamoli). Again, in majority of the cases, no major damage to the temple structure was reported.
The temple is believed to have been built in the 8th century A.D. in Nagara architectural style. The Shastras, the ancient texts on architecture, classify temples into three different orders; the Nagara or ‘northern’ style, the Dravida or ‘southern’ style, and the Vesara or hybrid style which is seen in the Deccan between the other two. The Nagara style’s primary feature is a central tower (shikhara) whose highest point is directly over the temple’s primary deity. This is oten surrounded by smaller, subsidiary towers (urushringa) and intermediate towers; these naturally draw the eye up to the highest point, like a series of hills leading to a distant peak. Setting the temple on a raised base (adhisthana) also shifts the eye upward, and promotes this vertical quality.
The type of damage in a masonry wall depends on the relative alignment between the direction of the ground shaking and the wall. If the ground shaking is perpendicular to the wall, the most commonly found cracks are vertical and if the ground shaking is parallel to the wall, the cracks are usually diagonal. Both of these lead to formation of different failure mechanisms – analysis of defect in design, quality and other parameters which led to the failure of the process. In the former case, an out–of–plane collapse mechanism is observed in which case the wall leaves may be detached or the entire wall may overturn. In the latter case, an in-plane-shear mechanism is realised and the entire column snaps.
Masonry structures typically show two types of mechanisms under earthquake ground shaking, namely global mechanisms and local mechanisms. Global mechanisms, which are typically in-plane shear response of structural walls, occur when the masonry structure is constituted of walls and floors/roofs that are well connected to each other. In-plane shear response in masonry walls is characterised by diagonal (x-cracks) or horizontal cracks. In most ancient masonry constructions, connections between structural elements and between roofs/floors and walls are poor, and these lead to out-of-plane mechanisms, or local mechanisms. Out-of-plane mechanisms are characterised by out-of-plane bulging/push out or overturning of walls, parapets and other free-standing elements.
The project started in 2013 with the aim of restoring the damaged parts of the temple and preventing damage against future events. There are a lot of challenges in structural assessments of heritage structures. Prior to a physical inspection of a historic structure as much information as possible about the structure must be gathered. Not only is it important to understand the actual structure of the historic building in question, but also the times in which it was built. Most historical structures have multi-leaf walls which are composed by one or more external and internal leaves. External leaves contain stonework or brickwork and internal leaves are usually made of rubble masonry or very weak infill materials, such as earth or loose material.
Dr. Menon mentions the lack of material characterisation of the inner and outer leaves of these walls. For example, the exterior and interior of the wall at Kedarnath temple are gneiss stone leaves and the infill is rubble stone, which is irregular in shape, size and structure. Since these are not appropriately characterised, it is difficult to carry out further investigation. Other challenges include unavailability of proper geometrical/architectural drawings and poor understanding of ancient construction practices, especially the sequence of construction: contact and connection between external and internal leaves.
Dr. Menon and his team have visited the temple four times between June 2014 and June 2015 for geophysical studies, structural investigations and preliminary structural health monitoring. There is a smile on his face as he mentions that they were allowed to enter the temple premises during the nights after all the pilgrims had left, through a small gate at the back. During their visits, the team used a lot of interesting equipment for analysis. An endoscope was used to analyse the walls of the temple and the inner structure was found to contain voids in the core masonry. The endoscope is very similar to the one a doctor uses to check the interior of the body. Another instrument, an infrared camera, which takes pictures of the radiation conditions, was used. It is similar to a common camera that forms an image using visible light. Instead, infrared camera forms images using infrared radiations. By recording surface temperatures under different exposure conditions, hidden voids and cavities were detected in the temple structure.
Structural analysis of a structure is done by employing various techniques. Different forces, deformations and accelerations are applied to the structure and its components to assess their effects since excess load may cause structural failure. These applied forces are called loads. One type of load is a dead load which includes loads that are relatively constant over time, including the weight of the structure itself, and immovable structures such as walls and plasterboards. The technique of subjecting the structure to dead load to assess its effects is called Gravity Load Analysis. When this was used on the temple, it was concluded that the structure has very high safety margin against gravity loads.
Another load which the temple’s structure was subjected to is the lateral load. These loads are live loads (temporary or of short duration such as the load due to wind) whose main component is a horizontal force acting on the structure. Most lateral loads vary in intensity depending on the structure’s geographic location, structural materials, height and shape. Dr. Menon’s team considered hydrostatic pressure to complete the lateral load analysis and found that the pressure has no damaging effect on the structure. And that the structure is safe against hydrostatic pressure. The temple was also subjected to gravity loading and a monotonic displacement-controlled lateral load pattern which continuously increases through elastic and inelastic behaviour until an ultimate condition is reached. This method of analysis is called Pushover Analysis. And again, it was concluded that the structure is safe against such loads. The pushover analysis is a method to establish the capacity of the structure. Pushover-based seismic assessment (comparing demands to capacity), showed that the structure was safe against lateral loads.
The magnitude of acceleration mentioned in the first paragraph is a measure of how hard the earth shakes at a given geographic point. It is known as the Peak Ground Acceleration (PGA). After all the analysis, it was found that the temple structure is safe against earthquake ground motion below 0.3g. It was also found that the timber sloped roof over the SabhaMandapa and inner mandapa (mandapas are halls in the temple) is a poor quality construction and is vulnerable to earthquake shaking.
The front gable wall (the triangular portion of a wall between the edges of intersecting roofs) has shown significant dislocation of stone blocks. To safeguard it from future earthquakes the reconstruction of the stone masonry supporting walls of the truss and gable walls has been proposed. The proposal includes introduction of timber band beams with timber cross ties along the four sides of the SabhaMandapa above the stone wall to ensure integral action of the entire structure in the event of earthquake shaking. The gable walls would also be provided with band beams to ensure greater out-of-plane resistance that is required under earthquake shaking.
Prof. V Kamakoti from the Department of Computer Science and Engineering at IIT Madras has worked on a wireless sensor network for structural health monitoring. It is desired to monitor the inclination/tilt of the temple structure over a period of time. The process is automated and periodic. The sensors will be installed at Kedarnath temple and will send periodic readings every fiteen minutes to a remote server located at IIT Madras. It is engineered to wake up upon an impact of 0.063g and is designed to work under sub-zero temperatures up to –30 degrees C. Network connectivity seems to be an issue. BSNL is being used which works from 10 am to 6 pm and there is no certainty that the power during the winter season when the temple is shut down for six months will remain on.
The work has been challenging for the team because of unpleasant weather at the site which is strikingly different from the weather in Chennai where the team stays for better part of the year. A lot of work needs to be done in the northern region around Zone 5. There is a 700 km central seismic gap in the Himalayan front which also includes Uttarakhand. A recent study (2015), claims that there is sufficient energy stored in the ongoing tectonic process which could generate earthquakes of high magnitudes as high as touching 8 on the Richter scale. But the time of the event cannot be predicted. It might occur tomorrow, or 50 years later. The only certain thing is it is going to happen and we need to be prepared.
Dr. Arun Menon is an Assistant Professor at the Department of Civil Engineering at IIT Madras, where he has been since 2010. He also currently serves as the Convener of National Centre for Safety of Heritage Structures (NCSHS). His research interests center on structural conservation of historical monuments which include seismic response, assessment and retrofitting of masonry structures, historical seismicity and seismic hazard analysis. He received his M.Tech. in Civil Engineering from IIT Madras. He received his M.Sc. and PhD from ROSE School, University of Pavia, Italy.
Sanket Wani is a final year student pursuing B.Tech. in Chemical Engineering at IIT Madras. He can usually be found browsing popular science content on the internet. Of late, he has developed an interest in science writing. He also takes a keen interest in eating at fancy restaurants and watching football.
All images in the article are courtesy of Dr. Menon. Cover Image Source: Debdutta Purkaysatha via Blogger